"We had broken-down trucks, non-armoured vehicles and ... we were carrying contaminated fuel.

"They are holding us against our will. We are now prisoners," she added.

'Isolated incident'

But according to a senior US military official, the unit involved had been ordered to carry out what is known as a maintenance stand-down.

The official said the soldiers involved were not under arrest or detained and he described the incident as isolated.

However, a US-led coalition spokesman in Baghdad told AP that a few of the troops had chosen to express their concerns in an "inappropriate manner" and caused a temporary breakdown in discipline.

The BBC's Nick Childs in the Pentagon says US convoy missions have been a main target of attacks by Iraqi insurgents, but are also vital to the US-led forces - so any breach of military discipline here is likely to be taken seriously.